Fast Framing Camera Research Articles

Comparative study of different Sn wetted W CPSs exposed to NBI fluxes in the OLMAT facility

Four different tin-wetted, tungsten CPS (Capillary Porous System) targets where exposed to NBI pulses in the OLMAT High Heat flux (HHF) facility. They include two flexible ones placed on a TZM support (W meshes and W felt) and two compact ones (sintered W disk and 3D printed W). A comparative study was performed using a fast-frame imaging camera and an infrared pyrometer. Surface temperature increase and homogeneity, particle ejection, CPS damage and overall behaviour were studied for each case. Sn drop/accumulation at the lower part of the targets was observed for all cases when they are heated up to around 400 °C except for the 3D printed W target that has its own Sn deposit. The 3D printed W target presented the best results in all aspects, withstanding heat pulses up to 58 MW/m2 in 100 ms without any damage or particle ejection. On the other hand, the W mesh targets displayed damage at 20 MW/m2 due to a bad thermal contact with the deposit, while the sintered W disk developed a crack during a series of 15 MW/m2 NBI pulses. As might be expected, a reduced increase of temperature during pulses is observed for the two compact W targets. The results and their relevance for the design of a Sn wetted W CPS for application as a DEMO-divertor material are discussed in the present work.

Fragment plume diagnostics for cryogenic pellet shattering studies: Development and first experimental results

Shattered pellet injection is the baseline technology for the ITER Disruption Mitigation System (DMS) [1]. ITER DMS requires the use of large, 28.5 × 57 mm (diameter x length) cryogenic pellets, made of hydrogen, neon, or a mixture of those gasses and accelerated to several hundred m/s velocity. Beside the production, acceleration and transfer of such ITER-size pellets, the characterization of the shattering is one of the main task of the ITER DMS Support Laboratory at center of Energy Research as part of the ITER DMS Task Force program to validate the design choices of the DMS [2]. For this purpose, a fragment plume diagnostic chamber and two optical diagnostics using fast framing cameras, namely the laser curtain at two distances from the shattering head and a shattering observation were designed and implemented. Preliminary experiments for different pellet material and velocities at 20⁰ and 30⁰ shattering angle showed a strong pellets sublimation during shattering process. The fragments seem to slide along the shattering plate, no significant total reflection was detected. No qualitative difference between 20⁰ and 30⁰ shattering angle was observed, but clear differences for different materials and or pellet speeds.

Low-Cost Laser-Acoustic PVC Identification System Based on a Simple Neural Network.

Desktop laser cutters are an affordable and flexible rapid-prototyping tool, but some materials cannot be safely processed. Among them is polyvinyl chloride (PVC), which users usually cannot distinguish from other, unproblematic plastics. Therefore, an identification system for PVC applicable in a low-cost laser cutter has been developed. For the first time, this approach makes use of the laser-ablative sound generated by a low-power laser diode. Using a capacitor microphone, a preprocessing algorithm and a very simple neural network, black PVC could be detected with absolute reliability under ideal conditions. With ambient noise, the accuracy dropped to 80%. A different color of the material did not influence the accuracy to detect PVC, but a susceptibility of the method against a color change was found for other materials. The ablation characteristics for different materials were recorded using a fast-framing camera to get a better insight into the mechanisms behind the investigated process. Although there is still potential for improvements, the presented method was found to be promising to enhance the safety of future desktop laser cutters.

Spectroscopic observation of super-Alfvénic field-reversed configuration merging process by mixing of tracer ions.

Visualization of the collisional merging formation process of field-reversed configuration (FRC) has been attempted. In the collisional merging formation process, two initial FRC-like plasmoids are accelerated toward each other by a magnetic pressure gradient. The relative speed of the collision reaches several times the typical ion sonic speed and Alfvénic speed. The magnetic structure of the initial-FRCs is disrupted in the collision process, but the FRC-like magnetic structure is reformed in ∼30 µs after the collision. Magnetic reconnection should occur in this process; however, general theoretical models in magnetohydrodynamics approximation cannot be applied to this process because of the high-beta nature of FRC and super-Alfvénic/sonic relative speed. In this work, the spectroscopic observation of the collisional merging FRC formation was conducted to evaluate the timescale and geometry of merging. A slight amount of tracer element (e.g., helium) was mixed into one of two initial-FRCs. Mixing of the tracer did not cause serious adverse effects on the performance of the initial-FRC in the collision and merging processes. The collision and merging processes were visualized successfully and observed using a fast-framing camera with a bandpass filter. The timescale of merging and the outflow speed in the collisional merging process of FRCs were optically evaluated for the first time.

Gas puff imaging system for edge plasma fluctuation measurements in large helical device.

A gas puff imaging system has been developed to measure edge fluctuations in large helical device. The optical system splits the image of the plasma into four wavelengths, Hα/Dα (656nm), HeI (1s2p-1s3d; 587.6; 1s2p-1s3d, 667.8; and 1s2p-1s3s, 706.5nm), enabling simultaneous measurement of the spatial distribution of line ratios of He I. The image of the plasma is amplified with an image intensifier and recorded with a fast-framing camera. The measurement area has a diameter of 20cm just outside of the last closed flux surface. The spatial resolution of the optical system is about 3mm, and the frame rate is 100kHz for acquisition of the four wavelength images. Signal-to-noise ratio is evaluated for the system, and further improvement is discussed. Clear images are obtained for all wavelengths and a slightly different pattern is recognized, depending on the wavelength. A singular value decomposition analysis can decompose the image clearly to one perpendicular and parallel to the magnetic field lines.

First Experiments and Radiographs on the MegaJOuLe Neutron Imaging Radiography (MJOLNIR) Dense Plasma Focus

A dense plasma focus (DPF) is a relatively compact coaxial plasma gun, which completes its discharge as a Z-pinch. These devices are designed to operate at a variety of scales to produce short (<100 ns) pulses of ions, X-rays, and/or neutrons. LLNL recently constructed and brought into operation a new device, the MegaJOuLe Neutron Imaging Radiography (MJOLNIR) DPF, which is designed for radiography and high-yield operations. This device has been commissioned and has achieved neutron yields of up to 3.8E11 neutrons/pulse at 2.5-MA peak current while operating at up to 1 MJ of stored energy in its original pulsed power configuration. MJOLNIR is equipped with a wide range of diagnostics, including nuclear activation detectors, neutron time-of-flight (nToF) detectors, a fast-framing camera, optical light gates, and a time-gated neutron and X-ray imager. LLNL also runs unique particle-in-cell (PIC) simulations of DPF discharges in the Chicago code and has gained significant insight into the various physical factors that influence neutron yield. MJOLNIR is one of the first DPFs whose design and continual upgrades are heavily influenced by model predictions. In this article, we describe insights from modeling, device operation, and recent results. Comparisons between modeling predictions and measurements, as well as X-ray and neutron images are presented.

Experimental Comparison between Event and Global Shutter Cameras.

We compare event-cameras with fast (global shutter) frame-cameras experimentally, asking: “What is the application domain, in which an event-camera surpasses a fast frame-camera?” Surprisingly, finding the answer has been difficult. Our methodology was to test event- and frame-cameras on generic computer vision tasks where event-camera advantages should manifest. We used two methods: (1) a controlled, cheap, and easily reproducible experiment (observing a marker on a rotating disk at varying speeds); (2) selecting one challenging practical ballistic experiment (observing a flying bullet having a ground truth provided by an ultra-high-speed expensive frame-camera). The experimental results include sampling/detection rates and position estimation errors as functions of illuminance and motion speed; and the minimum pixel latency of two commercial state-of-the-art event-cameras (ATIS, DVS240). Event-cameras respond more slowly to positive than to negative large and sudden contrast changes. They outperformed a frame-camera in bandwidth efficiency in all our experiments. Both camera types provide comparable position estimation accuracy. The better event-camera was limited by pixel latency when tracking small objects, resulting in motion blur effects. Sensor bandwidth limited the event-camera in object recognition. However, future generations of event-cameras might alleviate bandwidth limitations.

Effect of low temperatures and pressures on signal, lifetime, accuracy and precision of femtosecond laser tagging velocimetry

Femtosecond laser electronic excitation tagging velocimetry is experimentally characterized in pure nitrogen at pressures (0.228–101 kPa) and temperatures (72–298 K) relevant to high-speed ground testing. Evaluated metrics of performance include signal intensity/lifetime and velocity accuracy/precision. A fast-framing camera lens-coupled to a bandpass-filtered image intensifier acquired data at six principal time delays (0.1–5.0 µs). Results show signal intensity depends primarily on density and secondarily on temperature. Relative to atmospheric conditions, signal intensity is optimized at reduced densities and cryogenic temperatures. In agreement with previous experiments, the density dependence breaks down at extended time delays. Signal lifetime depends largely on density with a mild to moderate inverse correlation to initial signal strength. In contrast to previous work in pure nitrogen at above atmospheric densities, initial signal and lifetime exhibit different, more complicated trends at sub-atmospheric densities. Although accuracy lacks clear thermodynamic dependencies, precision strongly depends on density and temperature, largely driven by its more fundamental (though inverse) relationship to signal intensity. Precision also inversely depends on the length of the time interval used to determine velocity, in agreement with previous studies. Normalized accuracies and precisions of less than a percent and a few percent, respectively, are obtained in the supersonic free jet.

Cross-field transport in detached helium plasmas in Magnum-PSI

In this study, enhanced radial transport in a volume-recombining region in detached helium plasmas in a linear device, Magnum-PSI, was investigated. By installing a reciprocating Langmuir probe, electrostatic fluctuations with high spatiotemporal resolutions were measured and analyzed. As a result, the ion-flux profile broadening in the detached state and the coherent plasma structure, which has an internal electric field in the azimuthal direction, were confirmed. By analyzing the emission intensities obtained with a fast framing camera viewing around the probe head, an enhanced fluctuation, which has an azimuthal mode number of m = 1, was found to be correlated with radial plasma ejection. This m = 1 mode rotates by the drift with the radial electric field and magnetic field and is correlated with the m = 0 mode. These two modes behave like a predator and prey; they quasi-periodically appear with about a quarter-period shift. Because the ion flux flowing into the target plate decreases when the radial transport is enhanced, this cross-field transport disperses the ion flux and decreases the maximum heat load applied to the target.

Dust Formation from Arc Spots on Nanostructured Tungsten Surface

Arcing experiments were conducted in the linear plasma device Pilot-PSI, where a pulsed plasma was superimposed to a steady state plasma. The arcing was observed by a fast framing camera, and the sample was analyzed with a transmission electron microscope. Observations of glowing objects released from the sample in response to the arcing and destruction of the fuzzy layer at the edge of the arc trail without significant melting suggested that dust was formed and released from the surface in response to arcing.

Cathodoluminescent screen imaging system for seeded blob detection in toroidal plasma experiment.

We designed and built a diagnostic based on a cathodoluminescent screen for the detection of turbulent plasma structures with high spatial resolution. The screen is coated with a low threshold energy cathodoluminescent powder that emits light when exposed to a plasma. The emitted light is imaged with a fast frame camera combined with an image intensifier and an optical bandpass filter. The diagnostic is used to study turbulent structures and seeded blobs. The results are analyzed with pattern recognition algorithms to track the turbulent structures and study their evolution in time.

Application of dynamic mode decomposition to rotating structures in detached linear plasmas

Dynamic mode decomposition (DMD) was applied to time-series snapshots of dynamic behavior in detached plasmas with a fast framing camera in the linear plasma device NAGDIS-II. The DMD extracted radial plasma ejection and Er×B rotation structures, which are associated with blob-like plasma structures. Besides, we investigated the influences from neutral gas pressure on the growth rates of the DMD modes. By increasing the neutral gas flow rate in the detached plasma, the growth rate of rotation mode became larger while the frequency decreased. The results indicated that the ejected plasma existed for a longer time in the periphery region. It is likely due to the fact that the ratio of the radial velocity to the rotation velocity of the ejected plasma decreased.

Observation of shock-front separation in multi-ion-species collisional plasma shocks

We observe shock-front separation and species-dependent shock widths in multi-ion-species collisional plasma shocks, which are produced by obliquely merging plasma jets of a He/Ar mixture (97% He and 3% Ar by initial number density) on the plasma liner experiment [S. C. Hsu et al., IEEE Trans. Plasma Sci. 46, 1951 (2018)]. Visible plasma emission near the He-I 587.6-nm and Ar-II 476.5–514.5-nm lines is simultaneously recorded by splitting a single visible image of the shock into two different fast-framing cameras with different narrow bandpass filters (589 ± 5 nm for observing the He-I line and 500 ± 25 nm for the Ar-II lines). For conditions in these experiments (pre-shock ion and electron densities ≈5×1014 cm−3, ion and electron temperatures of ≈2.2 eV, and relative plasma-merging speed of 22 km/s), the observationally inferred magnitude of He/Ar shock-front separation and the shock widths themselves are <1 cm, which correspond to ∼50 post-shock thermal ion–ion mean free paths. These experimental length scales are in reasonable qualitative and quantitative agreement with results from 1D multi-fluid simulations using the chicago code. However, there are differences between the experimentally inferred and simulation-predicted ionization states and line emission intensities, particularly in the post-shock region. Overall, the experimental and simulation results are consistent with theoretical predictions that the lighter He ions diffuse farther ahead within the overall shock front than the heavier Ar ions.

Fast framing imaging and modelling of vapour formation and discharge initiation in electrolyte solutions

The formation of vapour and initiation of electrical discharges in normal saline and other NaCl solutions has been observed and modelled. Millisecond pulses of −160 to −300 V were applied to a sharp tungsten carbide electrode immersed in the liquid. A fast framing camera was used to observe vapour layer growth around the electrode by shadowgraphy. Images were also taken without backlighting to observe emission accompanying breakdown. The conductivity of the vapour layer has been estimated by comparison of experimentally measured impedances with impedances calculated by finite element modelling of the liquid and vapour around the electrode observed by shadowgraphy. The conductivity of the vapour layer is estimated to vary between ∼1 and 10−3 S m−1, which are orders of magnitude higher than expected for water vapour. The reason for the high conductivity is not clear, but may be due to injection of charge carriers into the vapour by corona discharge at the sharp tip and/or the presence of cluster ions in the vapour. Alternatively, the vapour layer may be a foam mixture of high conductivity liquid and low conductivity vapour bubbles. Discharge formation does not occur primarily at the sharp tip of the electrode, but rather at the side. Finite element models of the vapour layer show that the highest electric fields are found close to the sharp tip of the electrode but that the field strength drops rapidly moving away from the tip. By contrast slightly lower, but consistently high, electric fields are often observed between the side of the electrode and the liquid vapour boundary where plasma emission is mostly observed due to the shape of the vapour liquid boundary. The electron number density in the discharge is estimated to be ∼1021 m−3. There is evidence from the shadowgraphy for different boiling mechanisms; nucleate boiling at lower voltages and film boiling at higher voltages. A rule of thumb based on a simplistic model is suggested to predict the time to discharge based on electrode area, initial current, and electrolyte temperature, density, conductivity and heat capacity.

Comparative study of positive and negative pulsed discharge plasma characteristics of coaxial gun

The coaxial gun plasma generated by pulsed discharge possesses the characteristics of high speed and high density, and has potential application value in the field of fusion, space propulsion and astrophysics. In this paper, the effect of positive and negative pulsed discharges on plasma characteristics are investigated and a theoretical model for analyzing the morphology of positive and negative pulsed current sheets is proposed. Positive and negative pulsed discharges are realized by changing the direction of the rectifier diode in the pulse power supply to change the direction of the recharging current. Through theoretical analysis, and measurements by using photodiode, Pearson probe, magnetic probe, HD camera, fast-framing camera and RGB image processing, the plasmas generated by positive and negative pulsed discharges are compared and investigated. Most of experimental diagnoses concentrate on investigating the plasma behavior in the coaxial gun muzzle on a microsecond-order time scale. Because radial and axial transport characteristics of plasma change little, we think, the plasma characteristics in the muzzle still depend on the characteristics of plasma in the coaxial gun. Therefore, the conclusion of the theoretical analysis of the current sheet in the coaxial gun is still valid for the plasma in the muzzle. The theoretical analysis shows that the positive pulsed current sheet presents a parabolic shape and the negative pulsed current sheet displays a convex shape, which makes the negative pulsed current sheet sweep more efficiently and a large amount of plasma is concentrated near the inner electrode, namely the cathode, so the negative pulsed plasma is denser. For the positive pulsed plasma, near the inner electrode the plasma is thin and the magnetic pressure is powerful, and near the outer electrode, the plasma is dense and the magnetic pressure is weak. Therefore, the positive pulsed plasma is faster in movement speed but easier to split, and because of its dispersion, its transport stability is not so good as that of the negative pulsed plasma. The experimental results accord with the theoretical analyses. The final conclusion shows that under the same discharge parameters, the positive pulsed discharge produced plasma is faster in movement speed but more likely to split, and the negative pulsed discharge created plasma is denser in density and more stable. Therefore, for obtaining a higher density plasma, the negative pulsed discharge is recommended, and for achieving a high-speed plasma source, the positive pulsed discharge is advised to be adopted.

Particularities of shocks generated by underwater electrical explosions of a single wire and wire arrays

The results of experimental and numerical research of shockwaves generated by the underwater electrical explosion of a single wire and a planar wire array are reported. Experiments were conducted using a microsecond time scale pulse power generator delivering a <210 kA current with a <1 μs rise time. Streak and fast framing cameras were used to study the time- and space-resolved evolution of the shocks. The results show that (a) an aperiodic discharge constitutes the most efficient method of generating a shockwave; (b) in the case of a planar wire array, an almost simultaneous explosion can be obtained when the individual wire diameter is <100 μm (current density >2 × 107 A/cm2); (c) the planar shock velocity can be accelerated using the combustion properties of the Al wires efficiently, and (d) the shock velocity can be enhanced using a metal reflector placed below the array.

Evaluation of classic and 3D glass fiber reinforced polymer laminates through circular support drop weight tests

This study presents a new approach method for instrumented drop weight tests, with a central spherical impactor and a circular sample support. The tests were carried out on glass fiber reinforced polymer (GFRP) laminates with three types of reinforcement: plain-woven fabric, stitched plain-woven fabric and 3D non-crimp fabric (NCF). It was found that the plain-woven fabric reinforced laminate showed a more violent destruction mode (a massive fracture crack) in comparison with the other two. The stitched and the 3D NCF reinforced laminates exhibited a better impact resistance manifested by a higher stress at the failure initiation point, and a slower or totally arrested fracture crack growth. The applied methodology proved effective for the evaluation of materials under impact loading conditions. A novel element is the use of fast-frame camera images to measure the displacement of the specimens during the impact. It strongly improved the obtained results and enabled a reliable analysis of the strain-stress progress.

Multipoint measurements employing a microwave interferometer and a Langmuir probe in the detached linear plasma

Multipoint measurements were carried out by employing a microwave interferometer (MI) and a Langmuir probe (LP) in steady-state detached plasmas in the linear plasma device NAGDIS-II to reveal the structure of fluctuations along the magnetic field. We changed the LP position along the magnetic field while the MI was fixed at an upstream position. In addition, a fast framing camera was used to identify an azimuthal mode number, and the predominant mode number was identified as m = 1. By analyzing correlations between signals observed by the LP and the MI, it was found that a time delay of 10–20 kHz fluctuations gradually decreased toward the downstream direction. The results indicate a decrease in the rotation velocity in the E × B direction, and suggest that the 10–20 kHz fluctuation forms a spiral shape.

Fast camera imaging of plasmas in Alcator C-Mod and W7-X

In this work we present an analysis of fast-camera imaging of Alcator C-Mod's divertor region and of the plasma cross section in the W7-X stellarator. In C-Mod, the dynamics of intermittent edge plasma filaments are imaged at 397,100 frames per second in an oblique view. An automatic pipeline was developed to reconstruct the filament motion in the R, Z plane of the divertor using EFIT field line projections. Next, we added our fast-framing camera to W7-X's diagnostic suite for the 2017 run campaign, the first with a divertor. It has a tangential view of the machine cross section, including part of a divertor module. The view is coupled to the camera via a coherent fiber bundle. The spatial resolution is 2–4 cm and the typical plasma brightness limits the fastest usable frame rate to 30k frames/s. We observe filaments in the plasma boundary that rotate around the magnetic axis during radiative collapse; filaments were observed in steady state in the limiter configuration of W7-X by Kocsis et al. [1]. We also observe for the first time a 1–2 kHz quasi-coherent fluctuation in divertor target emissions in the W7-X “standard” magnetic configuration. Finally, a rough analysis of edge gas fueling efficiency using fast camera data and S/XB coefficients finds that fueling is less efficient at densities higher than 4 × 1019 m−3.

Fast-framing camera based observations of spheromak-like plasmoid collision and merging process using two magnetized coaxial plasma guns.

We have been conducting compact toroid (CT) collision and merging experiments by using two magnetized coaxial plasma guns. As is well known, an actual CT/plasmoid moves macroscopically in a confining magnetic field. Therefore, three-dimensional measurements are important in understanding the behavior of the CTs. To observe the macroscopic process, we adopted a fast-framing camera (ULTRA Cam HS-106E) developed by NAC Image Technology. The characteristics of this camera are as follows: a CCD color sensor, capable of capturing 120 images during one sequence with a frame rate of up to 1.25 MHz. Using this camera, we captured the global motion of a CT inside the magnetic field and the collision of two CTs at the mid-plane of the experimental device. Additionally, by using a color sensor, we captured the global change in the plasma emission of visible light during the CT collision/merging process. As a result of these measurements, we determined the CT's global motion and the changes in the CT's shape and visible emission. The detailed system setup and experimental results are presented and discussed.